A spin-polarized electron generating device is expected to be used in a spin-polarized low-energy electron microscope for observing a magnetic domain structure and in elementary-particle research in which elementary particles are generated by collision between protons and spin-polarized electrons. In these fields, particularly, high polarization and high external quantum efficiency are required. At present, a semiconductor photocathode is mainly used as a spin-polarized electron generating device. In a semiconductor photocathode, electrons are excited with irradiation light, and the excited electrons are extracted from its surface, thereby yielding an electron beam. In the case where the semiconductor photocathode is used as a spin-polarized electron generating device, by using circularly polarized light as excitation light, the spin of excited electrons is polarized; in this manner, the semiconductor photocathode functions as a spin-polarized electron generating device.
The principle of spin polarization in a GaAs semiconductor photocathode will be described. Circularly polarized light with which the semiconductor photocathode is irradiated excites electrons from a heavy-hole band and a light-hole band to a conduction band. At this time, electrons having different spins are excited from the bands at a ratio of 3:1. As a result, the spin of an electron beam output from the cathode is polarized, whereby a spin-polarized electron beam is obtained.
Regarding a spin-polarized electron generating device using a single layer of strained GaAs semiconductor, a technique disclosed in Patent Documents 1, 2, and 3 mentioned below is known. The technique improves spin polarization by imposition of strain on a GaAs semiconductor layer. In GaAs semiconductor, a heavy-hole band and a light-hole band are degenerated in the valence band at Γ point; thus, electrons are excited simultaneously from the two bands. If the valence band is split into the two bands by breaking degeneracy, spin electrons are excited from only one of the bands; thus, in principle, a spin polarization of 100% can be achieved. As for a band splitting method, there is a method of imposing strain on semiconductor. According to Patent Documents 1 to 3, on a GaAs substrate, a GaAsP crystalline active layer different in lattice constant from the GaAs substrate is epitaxially grown so as to impose strain in the GaAsP layer through lattice mismatch, whereby a spin polarization of 50% or higher is achieved through associated splitting of the valence band of GaAsP. Further, underlaying the active layer with GaAsP different in composition from the active layer, rather than forming the active layer directly on the GaAs substrate, is proposed for restraining degradation of crystallinity of the active layer.
Patent Document 4 mentioned below discloses a technique for improving spin polarization by use of a strained superlattice structure. A superlattice structure is such that semiconductor layers of two or more different band gaps, each layer having a thickness of 1 ML to several nm, are repetitively stacked together. In the case of electrons, a layer having lower energy level at the bottom of the conduction band is called a well layer; in the case of holes, a layer having higher energy level at the top of the valence band is called a well layer; and layers between which the well layer is sandwiched are called a barrier layer. Electrons and holes are confined in the well layer, whereby quantum levels are formed. Since the quantum level of heavy holes and that of light holes are formed in different energy regions, band splitting arises accordingly.
Further, a superlattice layer on which strain is imposed is called a strained superlattice structure. Through additive utilization of the strain and an effect yielded by quantum confinement, further band splitting is implemented. Patent Document 4 relates to a spin-polarized electron generating device having a GaAs—GaAsP strained superlattice structure formed on a GaAs substrate, and the spin-polarized electron generating device achieves a polarization of 90% or higher.
The technique disclosed in Patent Document 5 mentioned below relates to an electron generating device for generating a high-brightness polarized electron beam. For enhancement of brightness, an effective measure is to reduce the size of an electron beam generation region by convergence of excitation light. The invention of Patent Document 5 proposes a structure in which a photocathode is irradiated with excitation light from its back side, whereby the excitation light is converged by a lens without interference with a spin-polarized electron beam emitted from the front side of the photocathode. As for a semiconductor photocathode, Patent Document 5 discloses a newly invented GaAs—GaAsP strained superlattice structure formed on a GaP substrate which allows transmittance of excitation light. However, the structure is fails to provide high polarization.    Patent Document 1: Japanese Patent No. 3189444    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. H06-28970    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. H06-231676    Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 2000-90817    Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2007-258119